Magnetic resonance imaging (MRI) is a non-invasive medical procedure that utilizes magnets and radio waves to produce an image of the inside of a body. An MRI machine, or scanner, is capable of producing images of a body with excellent tissue contrast and without exposing the body to ionizing radiation (X-rays). In addition, MRI scans can see through bone and provide detailed images of soft body tissue.
A typical MRI machine includes a magnet that is utilized to create a strong homogenous magnetic field. A patient is placed into, or proximate, the magnet. The strong magnetic field causes atoms within the patient's body to align. A radio wave is directed at the patient's body, triggering atoms within the patient's body to emit radio waves of their own. These return radio waves create signals (resonant signals) that are detected by the MRI machine at numerous angles around the patient's body. The signals are sent to a computer that processes the information and compiles an image, or images. Typically, although not necessarily, an image is in the form of two-dimensional “slice” images.
Many medical procedures involve inserting medical devices (for example, catheters, guidewires, stents and drug delivery devices) into the body of a patient. Traditionally, X-ray fluoroscopy has been used for guidance of medical devices during intervention procedures, but this method has its drawbacks. MRI techniques are beginning to be used and investigated for obtaining an image of the medical device and body tissue that is proximate to the device. As a medical device is advanced through the body during an intervention procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device can be monitored to determine whether it has been placed properly and/or is functioning properly.
In the case of obtaining an MRI image of body tissue, an 1H MRI process is well suited. In the case of 1H MRI, the detectable species are protons (hydrogen nuclei). Most implantable or insertable medical devices are made of materials such as organic polymers, metals, ceramics, or composites thereof, which do not produce adequate signals for detection by an 1H MRI process. As such, a negative location on an MRI image may be used to indicate the location of the medical device. In such a case, the medical device is often difficult to see in an 1H MRI image, because it fails to produce sufficient contrast with respect to the surrounding body tissue or structures, and/or is too small to be readily detected.
The ability to identify the medical device in an 1H MRI image may be enhanced, passively, by incorporating paramagnetic or ferromagnetic contrast mediums into the device. Paramagnetic and ferromagnetic contrast mediums provide contrast between the medical device and the body tissue by altering the relaxation time of the MRI signal in the tissue near the agent. Another approach is an active approach that incorporates a coil onto the medical device and run a direct current (DC) through the coil. The current creates an adjustable inhomogeneity in the magnetic field that produces an artifact that indicates in an image the position of the medical device in the tissue. Another active approach is to incorporate a small radio frequency (RF) coil in the device. Activation of the RF coil enables the position of the coil to be shown in the MRI image.
Another type of MRI process is fluorine MRI where the detectable species are fluorine nuclei (19F). Fluorine MRI has been used in many different applications. For example, fluorine MRI enables the measurement of local oxygen pressure in body tissue because of the high soluability of oxygen in fluorocarbons, and so blood flow and vascular volume, for example, may be measured using a 19F MRI process. Other applications of 19F have been, for example, examining the biodistribution of fluorinated anesthetics and drugs.
So called “real time” MRI techniques are being developed as an alternative to X-ray tracking for tracking the location of an inserted or implantable device in an intervention procedure. In one such procedure, two images are obtained: 1) a tissue road map image is obtained using an external 1H MRI process; and 2) an 1H MRI field of view image is obtained using an MR catheter probe. The second image is displayed on the road map image. Other real-time MRI procedures use a single 1H MRI process, and rely on, for example, the use of a contrast agent so that the position of the medical device may be discerned in the 1H MRI image.
An MRI process that uses both 1H and 19F excitation has been used to perform in vivo 19F [1H] decoupling so that the presence of 19F nuclei susceptible to coupling with neighboring protons may be determined with better signal intensity and spectral resolution. This process may be performed using a time-sharing modulation method where 1H decoupling RF excitation pulses are generated after 19F RF excitation pulses and interleaved between successive sampling points where free induction decay following the 19F pulses is sampled.